For most of the months Dec-Feb (2010-2011) I recorded the times I left for work and the times I got there (excluding days that I stopped for gas etc).
I took almost exactly the same, route each trip, driving from my personal parking spot next to my apartment to the same spot in my office lot. I worked from home on snow days (except for one ... see the outlier). I usually covered the clock in my car so I could not be biased into a different driving pattern.
The curve fitting was done with my log fitting Matlab program (which you should all download if you ever need to fit a scatter plot of points to linear, exponential, logarithmic or power law curves). The slope of the line is 0.9589 which is very nearly 1. This means that for these months that traffic had a surprisingly small effect on the driving times.
photo seen previously on the blog: Cairo at night
This goes against my intuition and anecdotal experience of it taking longer to drive in when I leave during rush hour (8-9). I think part of the reason for this is that I took a good chunk of these data during the winter break of a few universities, so traffic would have been lighter than usual. Also, I don't have enough data to get a good error bars around these numbers. I'd expect traffic to be less predictable during rush hour.